Presently known hydraulic press mechanisms often consist of a single movable piston. In order to have an approach stroke, it is necessary, in that case, to displace all more hydraulic fluid as this stroke is greater. This can essentially be accomplished only by a pump, i.e., by a device which is complex with respect to the simplicity of the jack itself. Displacement of the fluid with a manual pump has to be effected using a large number of manoeuvers which of course make the operation slow and impractical.
Certain known devices provide a substantial and rapid approach displacement but require for this purpose a second jack and corresponding distribution chambers in order to transport the fluid.
Certain existing presses provide a fast approach stroke by means of the high flow rate of a low pressure pump, and a slow work stroke by means of the low flow rate of a high pressure pump. The feeding and control device, in the latter case, is costly with regard to the desired result.
Finally, certain devices are known which provide the same results at the price of greater complexity, for, in particular, control can only be effected using compressed air which must be introduced in several places of the device.